What the Heck is a Mud Motor

The use of mud motors evolved from vertical drilling in oil patches as drillers were forced to punch through bedrock. Adapting these tools into effective horizontal directional drilling (HDD) equipment required small tweaks to improve functioning, but the basic principles remain the same. Many HDD drillers know a mud motor is a tool for drilling through solid rock, but they may not know much else. This article will outline the specifics of what a mud motor is, how it works, and how these specialized HDD tools can increase efficiency and profitability.

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By Peter Melsheimer

The use of mud motors evolved from vertical drilling in oil patches as drillers were forced to punch through bedrock. Adapting these tools into effective horizontal directional drilling (HDD) equipment required small tweaks to improve functioning, but the basic principles remain the same. Many HDD drillers know a mud motor is a tool for drilling through solid rock, but they may not know much else. This article will outline the specifics of what a mud motor is, how it works, and how these specialized HDD tools can increase efficiency and profitability.

How Does a Mud Motor Work?

With a conventional duckbill or blade set-up, drilling through solid rock is a time-consuming process. When the person wants to change directions, they are forced to stop the rotation of the drill head and rely on the thrust of the rig and the pressure of the fluid as they use a slight rocking motion to painstakingly carve out a new trajectory.

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As its name implies, a mud motor derives its power from the drilling fluid that in a conventional set-up would normally be ejected. Instead, mud motors harness the power of the flow pressure to spin the bit, supplanting the force from the rig’s hydraulic motor.

When drilling in a straight line using a mud motor, every piece of the tool rotates. But when the operator wants to change directions, the mud motor’s design enables him or her to stop the drill stem, halting the mud motor’s spin. This results in the independent rotation of only the very end of the mud motor, where the bit is connected. The power section of a mud motor consists of a rotor and stator. These two components harness the pressure and flow of the drilling fluid and translate it into torque and rpm that spins the drill bit. Thus, a mud motor provides simultaneous rotary cutting action and steering.

How Does Steering Work?

With a conventional setup, the duckbill or blade is positioned at an angle. As the operator applies thrust, the blade deflects, causing the drill to change direction. A mud motor typically has a 2-degree bend, set back some two to three feet from the drill bit. By simply rotating the head until the bend reaches the intended direction, the operator can then stop the rotation of the drill carriage as the bit continues to spin. This is what enables accurate control over steering.

Additional HDD Equipment and Required Accessories

Because of their mechanics, mud motors require a large quantity of drill fluid. Be prepared to not only invest in a significantly higher amount of fluid, but to also use a recycler (or reclaimer) to pump out the mud, separate the solids, and return clean drilling fluid for re-use in the system.

Using a mud motor also means the operator will need to incorporate a high-flow transmitter housing into the drill string. The added fluid passages on a high-flow housing eliminate restriction so the fluid pressure doesn’t drop because of a bottleneck at the transmitter housing. Though this is an added expense, the high flow housing can also be used for conventional drilling. Bottom line: a high-flow housing is a necessity.

Finally, invest in a specialized bit for drilling solid rock. The common bit for use with a mud motor is a tricone. Three small balls surfaced with carbide studs work together to fracture the rock and then grind it into smaller chips. Though it’s a pricier investment than a standard bit, a tricone will significantly increase footage.

Final Thoughts Before Drilling

Unless drilling in solid rock all the time, renting a mud motor is likely the best course of action. Because mud motors are subjected to such stress, these versatile HDD tools have a wear life. They require basic service after 150 hours of use and a more detailed service after every 1,000 drill hours. Most operations send their mud motors out for repair rather than investing in the specialized equipment required to address maintenance in-house.

Success with a mud motor depends on doing things right. Trying to cut corners by buying a cheap rock bit, not using a high flow housing, or leaving the reclaimer out of the operation will compromise the entire job. This is not the time to skimp on tools or become impatient. Working with a mud motor might be slow going, but in the end, you will see increased productivity drilling through solid rock, making it well worth the investment. UP


About the Author: Peter Melsheimer is the marketing director at Melfred Borzall, an HDD tooling manufacturer. While the bulk of his time is spent creating marketing plans, Peter fills in wherever needed. He has been in the HDD industry since 1989 and by 1991 was demonstrating and educating the world about the advantages of horizontal directional drilling for utility installation. His formal education is in Mechanical Engineering, but has gained years of expertise in marketing learned on the job.

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